Nanofibrillar cellulose refers to isolated cellulose microfibrils or microfibril bundles derived from cellulose raw material. Nanofibrillar cellulose (NFC), which is also known as microfibrillar cellulose (MFC), and by other related names, is a natural polymer that is abundant in nature. Generally, nanofibrillar cellulose has high aspect ratio and the fibril length can be up to several micrometers.
Traditionally, nanofibrillar cellulose production techniques are based on grinding (or homogenization) of aqueous dispersion of pulp fibers. The concentration of nanofibrillar cellulose in dispersions is typically low, usually around 1-5%.
The production of nanofibrillar cellulose by fibrillating cellulose fibers into nano-scale elements requires intensive mechanical treatment. In order to produce purified cellulose or reduce energy demand, chemical treatment or enzymatic treatment may be applied prior or posterior to mechanical fibrillation.
Through the mechanical fibrillating process, the fibrillated cellulose achieves a “gel-point” after repeated passes. The gel-point is characterized by a critical point in the process at which the cellulosic suspension rapidly thickens to a more viscous consistency. In other words, the viscosity of the pulp fiber suspension increases dramatically during the process. Therefore, after the grinding or homogenization process, the obtained nanofibrillar cellulose material is a dilute viscoelastic hydrogel.
However, for certain applications, a challenge associated with using NFC/MFC is the high viscosity in aqueous state, for example for pickering stabilization and coating processes, wherein low viscosity is desirable. Pickering stabilization means the utilization of nanometer scale solid particles in stabilization of heterogeneous systems, like emulsions and foams.
Japanese published patent application JP2009-298972 describes a method for producing cellulose fibers as a gas barrier film material, said method comprising the steps of oxidation, microparticulation, UV irradiation and enzymatic treatment. PCT publication WO 2012043103 describes cellulose nanofibers being able to form a film with excellent oxygen barrier properties, said nanofibers being produced by the steps of oxidation and enzymatic treatment followed by mechanical treatment. Both publications involve three or more steps.
European patent application EP 2226414 A1 describes a process for producing cellulose nanofibers using a 4-hydroxy TEMPO derivative less expensive than TEMPO and a process capable of rapidly producing homogeneous cellulose nanofibers.
PCT application WO 2012/119229 A1 describes highly charge group modified cellulose fibers and the method for making the same. The method comprises the steps of providing a raw material cellulose, producing dialdehyde cellulose in a first chemical process and converting the aldehyde groups into charge groups in a second chemical process.
PCT application WO 2011/064441 A1 describes a method for manufacturing nanofibrillated cellulose pulp. In the method, an optical brightening agent is dosed into the raw material before/during the pre-refining/refining stage as an refining additive.
PCT application WO 2012/107643 A2 describes a method for fabricating fiber and film products and composites. In the method, an aqueous gel of nanofibrillar cellulose is introduced into the process of fabricating fiber and film products.
Contrary to the typical high aspect ratio nanofibrillar cellulose, there is another type of nanocellulose with low aspect ratio, cellulose whiskers, which can be obtained by subjecting cellulose fibers to extensive acid hydrolysis, through which cellulose fibers undergo transverse cleavage along the amorphous region and turn into cellulose nanowhiskers with low aspect ratio, which leads to low viscosity. However, the production yield trough acid hydrolysis is typically only 50-60% due to solubilization of amorphous regions upon hydrolysis. Thus, the cellulose nanowhiskers are considered remarkably more expensive materials compared to NFC/MFC-like products.
Therefore, there is a need of easy-to-produce and cost-effective nanowhisker-like materials having low viscosity and still preserving desirable properties of fibril cellulose.